The UN political chief has said Iran's delivery of weapons to the Syrian government is an apparent violation of UN sanctions banning arms exports by Iran.

Jeffrey Feltman raised the issue in today's monthly Middle East briefing to the UN Security Council.

Iran is a strong supporter of Syrian President Bashar al-Assad and has reportedly been supplying the government with weapons.

Mr Feltman said the Syrian government and opposition are focusing on the use of force.

He said the government is using heavy weapons on populated areas and the Syrian people are "suffering grievously from the appalling further militarisation of this conflict".

Mr Feltman said UN Secretary-General Ban Ki-moon has repeatedly expressed concern about the arms flows to both sides, which in the case of Iran appear to violate a 2007 resolution banning arms exports.

Mr Ban will attend a summit meeting of leaders of non-aligned developing nations in Tehran next week, with the Syrian crisis high on the agenda.

British Prime Minister David Cameron and US President Barack Obama have said the use or threat of chemical weapons in Syria was "completely unacceptable".

In a statement after a phone call tonight, the two leaders said the threat would force them to "revisit their approach so far".

They also discussed possible ways to bolster the Syrian opposition.

Elsewhere, a report in a Russian newspaper claims the Russian government believes Syria has no intention of using its chemical weapons and is able to safeguard them.

The report appeared to try to reassure the West that President Assad will not use chemical weapons against rebels.

A "confidential dialogue" with the Syrian government on the security of the arsenal has convinced Russia that "the Syrian authorities do not intend to use these weapons and are capable of keeping them under control themselves," the Kommersant newspaper reported.

It also quoted a Foreign Ministry official as saying the US had "firmly warned insurgents not to even come close to chemical weapons storage sites and production plants" and that "opposition groups are heeding" those demands.

"This shows that the West can exert very specific influence on Assad's opponents when it wants to do so," the official said.

Meanwhile, opposition activists have claimed that Syrian soldiers killed a Syrian journalist sympathetic to the revolt against President Assad during a raid in Damascus.

Mosaab al-Odaallah, who worked for the state-run Tishreen newspaper, was shot at his home by troops conducting house-to-house raids in the southern Nahr Eisha district of the capital.

It was not possible to verify their account.

Mr Odaallah was among at least 40 people killed by bombardment and other attacks in Damascus.

The Syrian army used tanks and helicopter gunships during the offensive against rebels.

The journalist was from the southern city of Deraa and had used a pseudonym to write online reports about the crackdown in his home town.

Massoud Akko, head of the public freedoms committee at the underground Syrian Journalists' Association, said Mr Odaallah's death brought to 54 the number of Syrian journalists, bloggers and writers killed by security forces during the 17-month uprising.